The present invention relates generally to video special effects generators for combining multiple video input signals into a single video output signal, and more particularly to a special effects generator utilizing digital circuitry for providing a gradual transition between two video input signals switched into the video output signals.
Digital special effects generators are known. One such digital special effects generator is described in U.S. Pat. No. 3,821,468 issued June 28, 1974 to Albert E. Busch and assigned to the same assignee as the assignee of the present invention and incorporated herein by reference. While digital special effects generators such as the one described in the above-mentioned Busch patent provide a variety of special effects, transitions in the video output signal between two video input signals are abrupt, and result in a display in which a sharp line separates two video signals that are displayed adjacent to each other on the viewing screen. Such sharp transitions are desirable for many special effects; however, for other special effects such as vignette-type inserts, it is desirable to have a gradual transition between the two video signals to provide a soft edge effect in which the two video signals gradually blend into each other when displayed on the screen.
Circuits for providing a gradual transition between video signals have been constructed, such circuits utilize standard special effects generators that have been modified by adding an analog video switch that gradually increases the amplitude of one of the video signals while simultaneously decreasing the amplitude of the other video signal to provide the soft edge effect. The video switch is generally controlled by an analog delay circuit interposed between the analog switch and the circuitry driving it, or by means of a digital function generator that is controlled by the digial special effects generator.
While these techniques provide a way of achieving a gradual transition between two video signals, when a symmetrical effect such as a circle or diamond effect is selected and a soft edge is desired, the switching delay introduced by the delay circuit driving the analog video switch causes the display of the central image (i.e. a diamond or circle) to shift the direction of scanning by an amount equal to the width of the transition border. The use of a digital soft edge transition circuit such as the one described in U.S. Pat. application Ser. No. 557,067 filed Mar. 10, 1975 by Albert E. Busch, et al. and entitled "Digital Soft Edge Video Transition System," now U.S. Pat. No. 3,941,925, eliminates the undesirable shift caused by the delay circuit. However, it is a characteristic of the above Busch circuit that as the central image is gradually enlarged to the point where the border of the central image approaches the edge of the viewing screen, the transition border abruptly disappears, thus giving an undesirable "snap effect" as the size of the central image is increased. Furthermore, the above Busch system provides a soft edge transition only in the horizontal direction.